The χ factor:: Determining the strength of activity in low-mass dwarfs

We describe a new, distance-independent method for calculating the magnetic activity strength in low-mass dwarfs, L-Halpha/L-bol. Using a well-observed sample of nearby stars and cool standards spanning spectral type M0.5 to L0, we compute chi, the ratio between the continuum flux near Halpha and the bolometric flux, f(lambda6560)/f(bol). This ratio can be multiplied by the measured equivalent width of the Halpha emission line to yield L-Halpha/L-bol. We provide chi values for all objects in our sample, and also fits to chi as a function of color and average values by spectral type. This method was used by West et al. to examine trends in magnetic activity strength in low-mass stars.